On Wednesday, gunmen shot at a woman working on the campaign in northwest Pakistan, killing her and her driver, one of five attacks during the day on polio workers. A male polio immunization worker was critically wounded in one of the shootings.

This week, six other people have been killed who were working on the immunization program, which has been jointly conducted with the Pakistani government. No one has claimed responsibility, but some Islamic extremists charge that the program is a cover for espionage.

At the UN, Secretary-General Ban Ki-moon condemned the killing as ‘‘cruel, senseless. and inexcusable.’’ He said the eight workers were among thousands across Pakistan ‘‘working selflessly to achieve the historic goal of polio eradication.’’

Sarah Crowe, spokeswoman for UNICEF, said the vaccination program has been suspended everywhere in Pakistan until an investigation by the Pakistani government is completed.

‘‘This is undoubtedly a tragic setback, but the campaign to eradicate polio will and must continue,’’ she said.

Some provincial governments in Pakistan continued to immunize children, independent of the UN drive.

Prevention efforts have managed to reduce the number of cases in Pakistan by around 70 percent this year compared to 2011, but the recent violence threatens to reverse that progress.

Suspicion for the attacks has fallen on the Pakistani Taliban because of their virulent opposition to the polio campaign, but the group’s spokesman, Ahsanullah Ahsan, denied responsibility.

Police say they have killed two militant suspects and arrested a dozen others in connection to the attacks but did not say whether they were Taliban.

Militants accuse health workers of acting as spies for the United States and claim the vaccine makes children sterile. Taliban commanders in the troubled northwest tribal region have also said vaccinations cannot go forward until the United States stops drone strikes in the country.

Insurgent opposition to the campaign grew last year after it was revealed that a Pakistani doctor ran a fake vaccination program to help the CIA track down and kill Al Qaeda founder Osama bin Laden, who was hiding in the town of Abbottabad in the country’s northwest.

The number of attacks this week on polio workers is unprecedented. They came as the government started a three-day vaccination drive Monday targeting high risk areas of the country, part of an effort to immunize millions of children under the age of five.